The present invention relates generally to an optical element in which an optical thin film is formed on a substrate, an optical apparatus, a film forming method, a film forming apparatus, and a device fabrication method, and more particularly to an optical element that includes a multilayer film made of fluorine is formed by sputtering, an optical apparatus, a film forming method, a film forming apparatus, and a device fabrication method.
Along with the recent demands on finer processing and improved economical efficiency, the further improvement of resolution and productivity of the projection exposure apparatus has been increasingly required. Use of a light source having a short wavelength is one method of improving the resolution. Recently, the practical use of a stepper starts which uses an excimer laser that oscillates light with wavelength shorter than a mercury lamp and is high-power as light source. An optical element in the stepper should form an optical thin film (antireflection film etc.) to reduce the light intensity loss by the surface reflection of a substrate etc.
In general, an optical element used for the light with a wavelength that is shorter than 400 nm is likely to cause, if its optical thin film is composed of a film material with a large light absorption factor or a film material with low laser durability, a light intensity loss by absorption, a substrate surface deformation and a film destruction by the heat absorption. Therefore, the optical thin film uses a film material with a low absorption factor of light and high laser durability, for instance, calcium fluoride such as magnesium fluoride (MgF2), lanthanum fluoride (LaF3), aluminum fluoride (AlF3), gadolinium fluoride (GdF3) and neodymium fluoride (NdF3). The substrate uses fluorine compound crystal such as calcium fluoride and quartz glass etc. A forming method of the film on the optical element is chiefly classified into an evaporation system and a sputter system. The evaporation system includes a vacuum vapor deposition, a molecular beam vapor deposition, and an ion plating and ion beam vapor deposition. The evaporation system evaporates a material (fluorine) that composes the thin film by heating, is coagulated on the substrate surface with a temperature that is lower than an evaporation temperature, and forms a fluoride thin film. The sputter system includes various methods, such as a conventional sputtering, a magnetron sputtering, an ion beam sputtering and ECR sputtering, ionizes the atoms and the molecules introduced into the chamber by the discharge, collides them with the target with high energy, springs the atoms out of the target surface, deposits the sprung atoms on the optical element (substrate), and forms a fluoride thin film (see, for example, Japanese Patent Application Publication No. 2001-279437 and Japanese Patent Application, Publication No. 11-223707).
However, these film forming methods make the temperature of the optical element to a high temperature, and deform the substrate surface. Therefore, a stress is caused between the substrate and the thin film such as fluoride thin film with a thermal expansion coefficient different from a thermal expansion coefficient of the substrate, and the thin film is exfoliated. Moreover, the film forming method of the sputter system makes fluoride by a chemical reaction of a metallic material (magnesium (Mg), lantern (La), aluminum (Al), and neodymium (Nd), etc.) as the target on the substrate. In this case, more gases including F2, H2O, and H2 are introduced than the film forming method of the evaporation system into the film forming chamber to unit a dangling bond. Moreover, in this case, even if H2O or H2 is not intentionally introduced into the camber, H2O or H2 is generated from an inner wall of the chamber etc. by discharging in the chamber. When the quartz etching, chemical species that derived from resultant fluorine or hydrofluoric acid is sandwiched in the interface between the film and substrate, an adhesion between the substrate and the film deteriorates and the thin film is likely to peel off. Moreover, the fluoride thin film that has a columnar film structure easily absorbs moisture in the atmospheric, and the moisture reacts with uncombined fluorine is sandwiched near the interface between substrate and the film and corrode the quartz substrate. Therefore, the film is likely to peel off. As a result of the film exfoliation, the optical element sometimes cannot satisfy the designed optical performance.